Development of a neutron source for a Monte Carlo-deterministic hybrid analysis of an accelerator-driven subcritical nuclear reactor (ADS)

Accelerator-driven systems (ADS) represent a promising technology for transmuting minor actinides and improving nuclear fuel-cycle sustainability. Their subcritical operation is sustained by an external neutron source generated by a high-energy proton beam impinging on a spallation target. The neutronic analysis of an ADS requires the description of both the high-energy spallation process and the subsequent neutron transport in the subcritical core. MCNP accurately models high-energy spallation, but cannot perform burnup calculations in fixed-source mode. ERANOS, instead, enables deterministic fuel-cycle and depletion analyses, but its available multi-group cross-section libraries are limited to energies below 20 MeV. A coupling procedure is therefore required to transfer the neutron source from the Monte Carlo calculation to the deterministic model. In this work, a hybrid Monte Carlo-deterministic methodology was developed for the analysis of a lead-cooled subcritical ADS driven by an 800 MeV proton beam impinging on a liquid metal target. MCNP was employed to simulate the spallation process, and a dedicated algorithm was developed to identify neutrons crossing the 20 MeV energy threshold. The extracted neutron events were then processed to generate the neutron source terms required by ERANOS for the deterministic calculation. The methodology was validated by comparing the ADS neutronic parameters obtained using the hybrid MCNP-ERANOS approach with dedicated full-core MCNP simulations. The effective multiplication factor differed by 102 pcm, while the average number of neutrons produced per fission differed by 0.06%. The total neutron yield per incident proton and the required proton beam current showed relative differences of 3.54% and 3.29%, respectively. The results demonstrate the reliability of the proposed methodology for hybrid MCNP-ERANOS neutronic analyses of ADS.